You may ask "why Maroona".
The typical length for Indian Pacific and Ghan is 774 meter.
The length of Ararat siding is less than 500, Maroona is 838.
None of the platforms along the line are long enough.
GSR would do as usual with passegers get on/off in the middle of a paddock.
For Maroona that means smooth out the gravel so it's comfortable for both passengers and the busses.
Same deal for Stawell and Lismore (or whichever thereabouts).
Also, as a high paying customer, I would not be impressed if the "Grampians Tour" was just a trip to Halls Gap and then off to Ararat.
I would want to see a lot more of the park, maybe exit lot further south.
Shunting onto the Portland line is only a Plan B, if the train parked is in the way.
Either because a super long configuration ( 800+) can't fit, or the mainline is so busy that Marooda must be used as a passing loop.
Neither is likely, but GSR do need contingency plans.
If that desperate, then suitable staff would be brought in to manage the train, and shunt back/ forth as needed to keep all lines clear.
North Geelong ? Don't think exrta couple of hours on bus is what the customer would put up with and not complain loud and wide.
It is mere speculation on my part, being the more likely scenarios to minimise unneccessary bus commuting, and maximise the tourist experience.